Tamara L. Townsend

On the Atlantic inflow to the Caribbean Sea

New observations are summarized that lead to the first comprehensive description of the mean inflow distribution in the passages connecting the Atlantic Ocean with the Caribbean Sea. The total Caribbean inflow of 28 Sv is shown to be partitioned approximately equally between the Windward Islands Passages (B10 Sv), Leeward Islands Passages ðB 8S vÞ; and the Greater Antilles Passages ðB10 SvÞ: These results are compared to a numerical model study using a 6-layer, 1=41 resolution Atlantic Basin version of the NRL Layered Ocean Model. Results from two simulations are described, including a purely wind-forced model driven by Hellerman and Rosenstein (J. Phys. Oceanogr. 13 (1983) 1093) monthly winds, and a model with an additional 14 Sv meridional overturning cell driven by inflow/outflow ports at the northern ð651N) and southern ð201S) model boundaries. The purely wind-driven version of the model exhibits a total Caribbean inflow of 17 Sv; consistent with expectations from steady, non-topographic Sverdrup theory. Nearly all of the wind-driven inflow occurs north of Martinique at latitude B151N. The net transport through the Lesser Antilles passages south of 151N (Grenada, St. Vincent, and St. Lucia passages) is nearly zero when the model is forced by winds alone. The addition of a 14 Sv meridional cell in the model increases the net Caribbean inflow to 28 Sv; with nearly all of the additional 11 Sv of inflow entering through the southern Lesser Antilles passages. The modeled inflow distribution resulting from the combined wind and overturning forced experiment is found to compare favorably with the observations. The seasonal cycle of the total inflow in the combined forcing experiment has a mixed annual/semiannual character with maximum in spring and summer and minimum in fall, with a total range of about 4 Sv: The seasonal cycle of the Florida Current resulting from this inflow variation is in good qualitative agreement with observations. Most of the seasonal inflow variation occurs through the Windward Islands passages in the far southern Caribbean, whose annual cycle slightly leads that of the Florida and Yucatan Currents. Variability of the modeled inflow on shorter time scales shows a dramatic change in character moving northward along the Antilles arc. The southern passages exhibit large fluctuations on 30–80 day time scales, which decay to very small amplitudes north of Dominica. Much of this variability is caused by North Brazil Current Rings that propagate northwestward from the equatorial Atlantic and interact with the abrupt island arc topography. The total range of transport variability in individual passages predicted by the model is consistent with observations. However, observations are presently too limited to confirm the seasonal cycles or variability spectra in the Caribbean passages. r 2002 Elsevier Science Ltd. All rights reserved.

Rings of the North Brazil Current: their structure and behavior inferred from observations and a numerical simulation

Large anticyclonic rings are shed from the retroflecting North Brazil Current (NBC) near 8°N in the tropical western Atlantic. New subsurface velocity and temperature measurements within three such rings are presented here and are found to be consistent with previous in situ and remotely sensed NBC ring measurements. A high-resolution numerical model of the Atlantic Ocean forced by monthly wind stress and an imposed meridional overturning cell is found to shed NBC rings that approximate those observed. The model rings are more surface-intensified than those observed and somewhat smaller in diameter. Both observed and modeled NBC rings move northwestward along the coast of South America with a speed of 8–16 cm/s, considerably slower than predicted by analytical theories describing westward ring propagation. At least 2–3 rings per year separate from the NBC retroflection. Annually, 1–3 rings translate intact from their formation region near 50°W to the islands of the southeastern Caribbean, where they disintegrate after a lifetime of about 100 days. The volume of fluid trapped within the core of an NBC ring and isolated from external mixing is estimated using potential vorticity as a tracer. The horizontal limits of the trapped core volume closely coincide with the radius of maximum swirl velocity, while the vertical limit of the core is typically less than the subsurface extent of significant swirl velocity. The core volume of a typical observed ring is 3.2±1.0×1013 m3. This corresponds to an annualized per-ring mass transport near 1 Sv (106 m3/s), similar to previous estimates. This study is the first to make use of subsurface temperature and velocity data to compute the volume of the anomalous ring core. NBC rings may be responsible for 3–4 Sv of direct mass transport across the equatorial-tropical gyre boundary or 20–25% of the total upper ocean cross-gyre transport required by the Atlantic meridional overturning cell. Translating NBC rings may contribute 20% of the total meridional heat transport by the ocean at this latitude.

Low-Latitude Circulation and Mass Transport Pathways in a Model of the Tropical Atlantic Ocean*

An eddy-resolving numerical ocean circulation model is used to investigate the pathways of low-latitude intergyre mass transport associated with the upper limb of the Atlantic meridional overturning cell (MOC). Numerical experiments with and without applied wind stress and an imposed MOC exhibit significant differences in intergyre transport, western boundary current intensity, and mesoscale ring production. The character of interaction between low-latitude wind- and overturning-driven circulation systems is found to be predominantly a linear superposition in the annual mean, even though nonlinearity in the form of diapycnal transport is essential to some segments of the mean pathway. Within a mesoscale band of 10‐100 day period, significant nonlinear enhancement of near-surface variability is observed. In a realistically forced model experiment, a 14 Sv upperocean MOC return flow is partitioned among three pathways connecting the equatorial and tropical wind-driven gyres. A frictional western boundary current with both surface and intermediate depth components is the dominant pathway and accounts for 6.8 Sv of intergyre transport. A diapycnal pathway involving wind-forced equatorial upwelling and interior Ekman transport is responsible for 4.2 Sv. Translating North Brazil Current rings contribute approximately 3.0 Sv of intergyre transport.

Modeled Sverdrup flow in the North Atlantic from 11 different wind stress climatologies

Abstract In studies of large-scale ocean dynamics, often quoted values of Sverdrup transport are computed using the Hellerman–Rosenstein wind stress climatology. The Sverdrup solution varies, however, depending on the wind set used. We examine the differences in the large-scale upper ocean response to different surface momentum forcing fields for the North Atlantic Ocean by comparing the different Sverdrup interior/Munk western boundary layer solutions produced by a 1/16° linear numerical ocean model forced by 11 different wind stress climatologies. Significant differences in the results underscore the importance of careful selection of a wind set for Sverdrup transport calculation and for driving nonlinear models. This high-resolution modeling approach to solving the linear wind-driven ocean circulation problem is a convenient way to discern details of the Sverdrup flow and Munk western boundary layers in areas of complicated geometry such as the Caribbean and Bahamas. In addition, the linear solutions from a large number of wind sets provide a well-understood baseline oceanic response to wind stress forcing and thus, (1) insight into the dynamics of observed circulation features, by themselves and in conjunction with nonlinear models, and (2) insight into nonlinear model sensitivity to the choice of wind-forcing product. The wind stress products are evaluated and insight into the linear dynamics of specific ocean features is obtained by examining wind stress curl patterns in relation to the corresponding high-resolution linear solutions in conjunction with observational knowledge of the ocean circulation. In the Sverdrup/Munk solutions, the Gulf Stream pathway consists of two branches. One separates from the coast at the observed separation point, but penetrates due east in an unrealistic manner. The other, which overshoots the separation point at Cape Hatteras and continues to flow northward along the continental boundary, is required to balance the Sverdrup interior transport. A similar depiction of the Gulf Stream is commonly seen in the mean flow of nonlinear, eddy-resolving basin-scale models of the North Atlantic Ocean. An O (1) change from linear dynamics is required for realistic simulation of the Gulf Stream pathway. Nine of the eleven Sverdrup solutions have a C-shaped subtropical gyre, similar to what is seen in dynamic height contours derived from observations. Three mechanisms are identified that can contribute to this pattern in the Sverdrup transport contours. Along 27°N, several wind sets drive realistic total western boundary current transport (within 10% of observed) when a 14 Sv global thermohaline contribution is added (COADS, ECMWF 10 m re-analysis and operational, Hellerman–Rosenstein and National Centers for Environmental Prediction (NCEP) surface stress re-analysis), a few drive transport that is substantially too high (ECMWF 1000 mb re-analysis and operational and Isemer–Hasse) and Fleet Numerical Meteorology and Oceanography Center (FNMOC) surface stresses give linear transport that is slightly weaker than observed. However, higher order dynamics are required to explain the partitioning of this transport between the Florida Straits and just east of the Bahamas (minimal in the linear solutions vs. 5 Sv observed east of the Bahamas). Part of the Azores Current transport is explained by Sverdrup dynamics. So are the basic path of the North Atlantic Current (NAC) and the circulation features within the Intra-Americas Sea (IAS), when a linear rendition of the northward upper ocean return flow of the global thermohaline circulation is added in the form of a Munk western boundary layer.

1/25 degree Atlantic Ocean Simulation Using HYCOM

Traditional ocean models use a single coordinate type to represent the vertical, but no single approach is optimal for the global ocean (Chassignet et al., 2000; Willebrand et al., 2001). Isopycnal (density tracking) layers are best in the deep stratified ocean, z-levels (constant fixed depths) provide high vertical resolution in the mixed layer, and terrain-following levels are often the best choice in coastal regions. The HYbrid Coordinate Ocean Model (HYCOM) (Bleck, 2002) combines all three approaches by dynamically choosing the optimal distribution at every time step via the layered continuity equation. This has lead to HYCOM being chosen for the next generation of ocean prediction systems both by NAVOCEANO and by NOAA.

Erratum to “On the Atlantic inflow to the Caribbean Sea” [Deep-Sea Research I 49 (2002) 211–243]

Erratum to ‘‘On the Atlantic inflow to the Caribbean Sea’’ [Deep-Sea Research I 49 (2002) 211–243] William E. Johns*, Tamara L. Townsend, David M. Fratantoni, W. Douglas Wilson Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA Naval Research Laboratory, Stennis Space Center, MS 39529, USA Department of Physical Oceanography, Woods Hole Oceanographic Institute, Woods Hole, MA 02543, USA Physical Oceanography Division, NOAA/AOML, 4301 Rickenbacker Causeway, Miami, FL 33149, USA